The present invention relates to gamma ray detectors having scintillation bodies made from plastic scintillator material.
Scintillation counters are radiation detectors that exploit atomic or molecular excitation produced by radiation passing through matter. Subsequent de-excitation generates photons which can be measured to give an indication of the energy deposited in the detector by the radiation. Various materials can be used for this process, but broadly speaking, scintillation materials fall into two categories. These are inorganic materials, such as sodium iodide doped with an activator such as thallium (NaI(Tl)), and organic materials, which include plastics such as polyvinyltoluene. Plastic scintillation materials are made by adding scintillation chemicals (phosphors) to a plastic matrix. This composition gives a scintillation material that has a very short decay constant. Thus, scintillation counters based on plastic scintillation materials have the advantage of a very fast decay time (of the order of a few nanoseconds) and hence fast response, whilst also being relatively inexpensive.
Plastic scintillation counters are widely used for the detection of charged particles and neutrons in a variety of applications. However, this type of detector is not normally appropriate for the detection of gamma radiation since, for such applications, one requires a material having a high density and high atomic number. Also, plastic scintillation counters are poor at distinguishing between gamma rays of different energy. Nevertheless, very large plastic scintillation counters are currently used in applications in which it is important to detect a weak source of gamma radiation without a need to recognise the particular radioisotope that is generating the radiation. In other words, the energy of the gamma radiation is not of interest. For example, when a nuclear site is decommissioned, a final survey may be performed using large area plastic scintillators to ensure that all significant sources of radioactivity have been removed. For this purpose, it is not essential to be specific about the nature of the source.
There are a number of other situations in which it is important to detect the presence of radioactive materials very rapidly. For example, this may occur when such materials are carried by a person passing through a security check at an airport, or in a vehicle crossing a border check-point [1]. As a consequence, portal monitors that are able to detect a weak radioactive source (few μCi) within a fraction of a second have been constructed by a number of manufacturers (for example: TSA Systems, Ltd of 14320 Longs Peak Court, Longmont, Colo. 80504, USA; Polimaster Ltd of 112 M Bogdanovich Street, Minsk 220040, Republic of Belarus; Canberra Industries of 800 Research Parkway, Meriden, Conn. 06540, USA; and SAIC of 10260 Campus Point Drive, San Diego, Calif. 92121, USA). These detectors employ large area plastic scintillation counters that have dimensions of typically 200 cm by 7.6 cm by 7.6 cm. The optical signal from a scintillation event in such a detector is sensed using a photomultiplier optically coupled to the plastic. The light collection efficiency of many detectors used for this purpose is poor, so in some cases coincidence techniques are used to discriminate a genuine signal from system noise.
A disadvantage of the use of large portal monitors to scan commercial vehicles is the inability to distinguish between naturally occurring isotopes such as those in containers of glazed ceramic tiles and bananas, and those related to the illegal transport of nuclear materials or commonly used in medical diagnosis and treatment applications. The impact on the operation of a busy port of false identification of radioactive materials in a cargo can be highly detrimental. As a consequence, there is a need for detector systems which are both very sensitive and also able to identify particular isotopes, with the object of distinguishing naturally radioactive materials from illicit materials. One proposed approach is to record the ratio of the number of counts in several broad energy windows, but this provides only a crude indication of the identity of the source that has been detected by a large plastic portal monitor.